Actually, printed a prototype of contrarotating props, will see if I can find the pictures and post soon. , not yet made, but I have the drawings and parts needed, bearing gears, etc… But need to make some parts of steel and aluminum to make it work. 3D printed parts will never last:P
it uses the gears shown, and two seals on each contrarotating axle, same gearbox system used on small torpedoes. I have a more complete model with all the bearing, seal, gear, details, it adds about 70mm to the propulsion unit. Can be made smaller.
Only found this video on my quick search, had to uploaded it to YouTube to make it work.
Also found this quick and dirty sketch, ps it is written in Norwegian:P